Add missing math delim decorations

This avoids a message "Deco was not found. Programming error?" on stderr.
The added decorations are defined by amsmath, and equivalent to \vert (single
vertical bar) and \Vert (double vertical bar), respectively. They are used to
distinguish the single and paired versions (for use with \left and \right).
These symbols should cause amsmath to be loaded, but this would be too
dangerous to implement now.

src/mathed/MathSupport.cpp

@@ -315,7 +315,11 @@ named_deco_struct deco_table[] = {
 	{"/",              slash,      0 },
 	{"slash",          slash,      0 },
 	{"vert",           vert,       0 },
+	{"lvert",          vert,       0 },
+	{"rvert",          vert,       0 },
 	{"Vert",           Vert,       0 },
+	{"lVert",          Vert,       0 },
+	{"rVert",          Vert,       0 },
 	{"'",              slash,      1 },
 	{"<",              angle,      0 },
 	{">",              angle,      2 },